Joint replacement surgery is quite common and it enables many individuals to function normally when they otherwise would not be possible to do so. Typically, an artificial joint includes metallic, ceramic and/or plastic components that are fixed to existing bone. One of the more common joints that undergoes replacement surgery is the knee. Knee arthroplasty is a well known surgical procedure by which a diseased and/or damaged natural knee joint is replaced with a prosthetic knee joint. A typical knee prosthesis includes a femoral component, a patella component, a tibial tray or plateau and a tibial bearing insert coupled to the tibial tray. The femoral component generally includes a pair of laterally spaced apart condylar portions that have distal surfaces that articulate with complementary condylar elements formed in a tibial bearing insert.
Total knee prostheses can essentially be classified into three basic categories based on the techniques and components involved in the surgery. In a first category, the articular surface of the distal femur and proximal tibia are “resurfaced” with respective condylar-type articular bearing components. These knee prostheses provide substantial rotational and translational freedom and require minimal bone resection to accommodate the components in the available joint space. The patella-femoral joint may also be resurfaced by a third prosthetic component, as well. The femoral, tibial and patella prosthetic resurfacing components are affixed to respective adjacent bone structure by a cementing or by a biological bone ingrowth fixation means or any other suitable technique.
The femoral component provides medial and lateral condylar bearing surfaces of multi-radius design of similar shape and geometry as the natural distal femur or femoral-side of the knee joint. The tibial component can be made entirely of plastic (UHMWPE: ultra-high molecular weight polyethylene) or it can be made of a metallic base component and interlocking plastic component. The plastic tibial bearing surface can be of concave multi-radius geometry to more or less match the mating femoral condyles. Both the femoral and tibial components are independently positioned on either side of the knee joint and are not mechanically connected or linked together, as in the case of hinged type of knee prostheses, which constitutes the secondary category of total knee prostheses.
In resurfacing types of total knee prostheses according to the first category, the tibial bearing surface geometry can assume a variety of configurations, depending upon the desired extent of articular contact congruency and associated translational (medial-lateral and anterior-posterior) and rotational (axial and varus-valgus) secondary femoro-tibial motions. These various secondary motions allow the resurfaced knee to function in a natural-like biomechanical manner in conjunction with the surrounding ligamentous and muscle structures about the knee joint. The soft tissue structures maintain the femoral and tibial bearing surfaces in contact, provide the necessary levels of force constraint to achieve knee joint stability, and functionally decelerate the principal motion in flexion-extension and secondary motions, such as axial rotation, in a controlled manner. Additionally, this functional interaction between the surrounding tissue structures and the implanted knee prosthesis minimizes abrupt motion stoppage or impact loading of the prosthetic articular surfaces, thus preventing overstressing at the component fixation interface.
According to the second category, a mechanically linked, or hinged type of knee prosthesis provides a fixed fulcrum flexion-extension capability. The “hinged knee” therefore is usually surgically indicated in selected cases where the surrounding soft tissue structures are grossly degenerated and incapable of providing functionally acceptable knee joint stability.
The third category of total knee prosthetic devices, the posterior stabilized total knee provides more predictable kinematics than the first category. The posterior-stabilized total knee devices essentially incorporate all of the functional features of the first category, that is, the resurfacing condylar-type of knee prostheses, in addition to incorporating a mechanical cam/follower mechanism for providing posterior (tibia-to-femur) constraint. The cam/follower mechanism is positioned within the intercondylar space of the femoral component and provides substitutional posterior constraint, as a predesigned compensation feature for lost posterior cruciate function or for compromised posterior knee stability. This cam/follower mechanism enables the femur to ‘roll-back’ on the tibia providing a mechanical advantage to the quadriceps during flexion.
The cam portion of the cam/follower mechanism, generally includes a convex lobe shaped surface, integrally machined or cast within a box-like structure known as the “stabilizer box,” located between the medial and lateral condyle bearing surfaces of the femoral component as shown in FIG. 1. The stabilizer box can also be referred to as being an intercondylar portion of the femoral component. The cam surface is generally formed within the posterior wall portion of the stabilizer box and is bounded by the superior wall on the top, the medial and lateral wall portions on the sides and the anterior portion. The stabilizer box structure, thus formed, occupies a significant envelope, relative to the overall dimensions of the femoral component and therefore, requires a substantial resection of viable bone to allow its accommodation within the intercondylar sector of the distal femur.
The posteriorly positioned articular convex surface of the cam is precisely ground and highly polished. The convex cam articulates with the anteriorly positioned and posteriorly oriented follower, as the knee undergoes femoro-tibial flexion. The mating follower surface is typically machined integral within the ultra-high molecular weight polyethylene (UHMWPE) tibial component. The follower member usually consists of a relatively convex or flat articular surface located on the posterior side of an upwardly extending post-like structure, which is positioned between the concave medial and lateral tibial plateau bearing surfaces. The resultant action of the contacting cam/follower mechanism provides posterior stabilization or constraint of the tibial component, relative to the femoral component: generally from about mid-range to full range of flexion. Within this limited range, therefore, the stabilizing mechanism essentially simulates the functional contribution of the natural posterior cruciate ligaments attached between the anterior femur and posterior tibia aspects of the knee joint. Additionally, since the cam/follower surface geometry is generally non-congruent, the mechanism can be designed to produce posterior roll-back of the femoro-tibial articular contact, simulating the natural biomechanical displacement characteristics of the natural knee.
Examples of posterior-stabilized total knee prostheses of the type described above, are disclosed in U.S. Pat. Nos. 4,209,861 to Walker; 4,298,992 to Burstein et al.; 4,213,209 to Insall et al.; and 4,888,021 to Forte et al. Each of the devices described in the above patents incorporates a UHMWPE tibial component with a pair of medial and lateral concave plateau bearing surfaces and a metal alloy femoral component with mating multi-radius condylar runners which ride on the bearing surfaces. The articulation of the femoral condyles with the tibial plateau bearing surfaces allows primary femoro-tibial flexion and extension, and secondary (freedom) motions of axial and varus-valgus rotations and anterior-posterior and medial-lateral translations. The knee joint reaction forces during primary or secondary motion are principally supported by the tibial bearing surfaces, and to some extent by the cam/follower surfaces, and are transferred to the underlying fixation interfaces and adjacent supportive bone structures.
Additionally, the UHMWPE tibial component incorporates an upwardly extending post-like structure which is positioned between the plateau bearing surfaces, slightly anterior of the component mid-line. The generally convex or flat follower surface is integrally machined on the posterior-side of the post. With the femoral and tibial knee components in a normally reduced, surgically implanted position, the upwardly extending tibial post extends into the stabilizer box structure located within the intercondylar space of the femoral component. Posterior tibial constraint is achieved when the posteriorly oriented face of the follower contacts the generally anteriorly oriented lobe surface of the cam.
However, there are a number of disadvantages with the geometries of conventional posterior cruciate substituting knee designs. In particular, one common complaint among knee surgeons is that posterior cruciate substituting knee replacements remove too much bone. Excessive bone removal can lead to intraoperative intercondylar fractures due to the stress concentration created by cutting out bone to accommodate the box of the design. Bone removal is also not desired in that in the event of revision surgery, the more bone available, the easier the revision surgery will be. It is therefore desirable and there is a need for an improved posterior cruciate substituting knee design that minimizes the amount of bone that is needed to be removed.
Another limitation with conventional posterior cruciate substituting knee designs is that the retrieved knee replacements show consistent deformation patterns in particular locations on the central post of the tibial insert that is typically made from UHMWPE. A common location for the damage to the tibial insert is the anterior face of the post. This deformation is often in the form of a “bowtie” pattern and is the result of the continued interaction of the implant components over time and likely occurs when the patient hyperextends their knee. In rare cases, this deformation can contribute to gross mechanical failure of the post. In view of the foregoing, there is a need for an improved posterior cruciate substituting knee design that reduces the stresses that contribute to this pattern of deformation.